Electrochemically modified Corey-Fuchs reaction for the synthesis of arylalkynes. the case of 2-(2,2-dibromovinyl)naphthalene

Article abstract of DOI:10.3762/bjoc.14.76

The electrochemical reduction of 2-(2,2-dibromovinyl)naphthalene in a DMF solution (Pt cathode) yields selectively 2-ethynylnaphthalene or 2-(bromoethynyl)naphthalene in high yields, depending on the electrolysis conditions. In particular, by simply changing the working potential and the supporting electrolyte, the reaction can be directed towards the synthesis of the terminal alkyne (Et₄NBF₄) or the bromoalkyne (NaClO₄). This study allowed to establish that 2-(bromoethynyl)naphthalene can be converted into 2-ethynylnaphthalene by cathodic reduction.

Full text of DOI:10.3762/bjoc.14.76

Electrochemically modified Corey–Fuchs reaction for
the synthesis of arylalkynes. The case of
2-(2,2-dibromovinyl)naphthalene
Fabiana Pandolfi, Isabella Chiarotto and Marta Feroci*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2018, 14, 891–899.
Deptartment of Fundamental and Applied Sciences for Engineering
(SBAI), Sapienza University of Rome, via Castro Laurenziano, 7,
00161, Rome, Italy
doi:10.3762/bjoc.14.76
Received: 27 December 2017
Accepted: 12 April 2018
Published: 23 April 2018
Email:
Marta Feroci* - marta.feroci@uniroma1.it
This article is part of the Thematic Series "Electrosynthesis II".
Guest Editor: S. R. Waldvogel
* Corresponding author
Keywords:
arylalkyne; carbon–bromine bond cleavage; cathodic reduction;
Corey–Fuchs reaction; 2-(2,2-dibromovinyl)naphthalene
© 2018 Pandolfi et al.; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
The electrochemical reduction of 2-(2,2-dibromovinyl)naphthalene in a DMF solution (Pt cathode) yields selectively 2-ethynyl-
naphthalene or 2-(bromoethynyl)naphthalene in high yields, depending on the electrolysis conditions. In particular, by simply
changing the working potential and the supporting electrolyte, the reaction can be directed towards the synthesis of the terminal
alkyne (Et4NBF4) or the bromoalkyne (NaClO4). This study allowed to establish that 2-(bromoethynyl)naphthalene can be con-
verted into 2-ethynylnaphthalene by cathodic reduction.
Introduction
Terminal alkynes, due to the considerable triple-bond strength The recently published papers confirm the present interest in the
(839 kJ mol−1), are characterized by a moderate thermo- chemistry of terminal alkynes, e.g., in the synthesis of sulfin-
dynamic reactivity [1]. Nevertheless, both the C–C triple bond amides and isothiazoles [4], 1,3-enynes [5], α-monosubstituted
and the terminal C–H bond can be efficiently and selectively propargylamines [6], 2-substituted pyrazolo[5,1-a]isoquin-
activated by metal or metal-free catalysts. Therefore, terminal olines [7], etc.
alkynes can be considered as raw material (thus an important
resource).
Terminal alkynes can be prepared by dehydrohalogenation of
vicinal dihalides or vinyl bromides using sodium in ammonia or
The use of terminal alkynes, activated by catalysts, as building strong bases [8]. Alternatively, the compounds are accessible by
blocks or intermediates in the synthesis of a large number of homologation of aldehydes following the Bestmann modifica-
chemicals is extensively summarized in recent reviews [1-3]. tion of the Seyferth–Gilbert reaction, using in situ generated
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Beilstein J. Org. Chem. 2018, 14, 891–899.
dimethyl (diazomethyl)phosphonate [9,10]. Moreover, the alde- could be performed under mild conditions and in the absence of
hyde homologation to terminal alkynes can also be obtained reducing agents or bases in the reaction mixture.
using the Corey–Fuchs reaction [11]. This is a two-step reac-
tion in which an aldehyde is at first converted into a 1,1-dibro- Electrochemical methods can be considered an environmental-
moalkene with chain extension by one carbon atom through the ly friendly technique: they rely on the use of practically mass-
reaction with carbon tetrabromide and triphenylphosphine less electrons (which are not converted to byproducts) instead
(Scheme 1, reaction 1). The second step comprises the conver- of stoichiometric amounts of redox reagents and frequently
sion of the 1,1-dibromoalkene into the corresponding alkyne by these reactions are carried out at room temperature and at
reaction with BuLi at −78 °C in THF (Scheme 1, reaction 2) atmospheric pressure, etc. [16-19].
[12].
The electrochemical behavior of halogenated compounds has
Recently, a chemical modification of the second step of the been extensively investigated [20-22]. The cleavage of the
Corey–Fuchs reaction was reported, in which the authors used C–halogen bond, yielding (via a radical intermediate) the corre-
Cs2CO3 as the base and performed the reaction in DMSO at sponding carbanion and halogen anion, can be achieved by a
115 °C for 12 h [13]. Good to high yields of terminal alkynes bielectronic cathodic process (Scheme 2). The electrolysis is
were obtained (50–98%). Also DBU (4 equiv) in MeCN at carried out at a suitable controlled potential, i.e., at a potential
room temperature is effective to carry out the second step of the that is negative enough to achieve the selective fission of the
Corey–Fuchs reaction, affording good to high yields of envisaged C–halogen bond [23].
arylalkynes. In the latter reaction DBU acts both as base and as
organocatalyst [14]. In all cases, an excess of a strong base or Therefore, the reactive species is an electrochemically gener-
high temperature are necessary for the reaction to proceed. An ated carbanion and the outcome of the reaction strongly
overview on the importance of the Corey–Fuchs reaction for the depends on the complex reactivity of this intermediate. More-
synthesis of natural products has been pointed out by Heravi over, this reactivity is influenced by the reaction conditions,
and co-workers recently [15].
such as the solvent, supporting electrolyte, electroinactive sub-
strates, temperature, working potential and amount of con-
As mentioned above the second step of the Corey–Fuchs reac- sumed charge [24].
tion requires the cleavage of a C–Br bond. We thus envisaged if
this could be achieved electrochemically via a selective Our group intensively investigated the electrochemical behav-
cathodic cleavage of the C–Br bond. In this way, the reaction ior of 1,1-dibromoalkenes by means of cyclic voltammetry and
Scheme 1: The Corey–Fuchs reaction.
Scheme 2: Electrochemical reduction of a carbon–halogen bond.
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electrolyses [25] and we reported the selective synthesis of been demonstrated to be a selective inactivator of cytochrome
vinyl bromides through the cathodic reduction of 1,1-dibro- P-450 2B4 [26] and an inhibitor also of other cytochrome P-450
moalkenes in the presence of acetic acid. The electrolysis condi- isoforms [27]. We thus decided to carry out our study
tions in this transformation were optimized in order to avoid or using 2-(2,2-dibromovinyl)naphthalene (1a) as starting
minimize the formation of the terminal alkyne. The latter was material for the synthesis of 2-ethynylnaphthalene (2a,
obtained as the major product in the absence of a proton donor Scheme 4).
and its formation could be suppressed when performing the
reaction with a Au cathode in acetonitrile (ACN) as the solvent
and in the presence of an excess of acetic acid as the proton
source. Under these conditions good yields of the vinyl bro-
mides were obtained with preference of the Z-isomers
(Scheme 3).
Scheme 4: Scope of this work.
Results and Discussion
In our previous work [25], we found that the cathodic reduction
of 2-(2,2-dibromovinyl)naphthalene (1a), carried out at the
Scheme 3: Electrochemical synthesis of vinyl bromides [25].
potential of the first voltammetric peak in ACN on a Au
cathode and in the presence of an excess acetic acid, yielded the
We have now reconsidered this investigation in order to obtain corresponding vinyl bromides (Scheme 3) in 75% yield
terminal alkynes and to avoid the formation of vinyl bromides. (Z/E 82:18). The main product was 2-ethynylnaphthalene (2a,
The scope of this paper is the determination of the electrolysis 65%) when the electrolysis was carried out in the absence of
conditions for the transformation of 1,1-dibromoalkenes into the acetic acid as protonating agent (1.8 F consumed charge). Due
corresponding terminal alkynes, in order to carry out the second to the importance of the latter product, we decided to recon-
step of the Corey–Fuchs reaction under milder conditions.
sider this procedure in order to direct the synthesis towards the
formation of the alkyne. We have therefore reconsidered the
2-Ethynylnaphthalene (2a) is a small molecule with a high and voltammetric behavior of 1a at Pt, Ag and GC cathodes in DMF
selective biological activity. In particular, this molecule has or ACN solutions (Figure 1).
Figure 1: Voltammetric curves of 1a 0.020 mol dm−3; Pt, glassy carbon (GC) or Ag cathode. ν = 0.2 V s−1, T = 25 °C; solvent left: DMF/Et4NBF4
0.1 mol dm−3; right: ACN/Et4NBF4 0.1 mol dm−3.
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The voltammetric curves of 1a show the presence of different reactions – when compared to the result obtained using a Au
reduction peaks which are affected by the solvent and by the cathode in the previous paper) [25].
electrode material (see peak potential Table S1 in Supporting
Information File 1). These voltammograms (and the data re- In order to ascertain the role of the solvent in this electrosyn-
ported in Supporting Information File 1, Table S1) evidence the thesis, we carried out an electrolysis in DMF instead of ACN on
catalytic effect of the silver cathode in the C–Br bond reduction a Pt cathode at the controlled potential of −2.00 V vs SCE, cor-
(Ep1 is quite less negative on Ag cathode) [28,29], although this responding to the first reduction wave of 1a (Table 1, entry 2).
effect is more evident in DMF than in ACN. In any voltammo- The current flow was stopped after the disappearance of 1a
gram, the cathodic peak at the less negative potential should be (3.0 F). Also in this case the only product was the expected
related to the cleavage of the C–Br bond. In order to confirm alkyne 2a with a higher yield (80%).
this statement, we carried out a first electrolysis in acetonitrile
on a Pt cathode at the controlled potential of −1.75 V vs SCE, An increase in the charge did not lead to an increase of the yield
corresponding to the first reduction wave of 1a (Table 1, (81%, Table 1, entry 3). When the working potential was in-
entry 1). The current flow was stopped after the disappearance creased to −1.75 V and the electrolysis was stopped after the
of 1a (6.0 F). The only product was the expected alkyne 2a total consumption of 1a (1.5 F), a mixture of products was ob-
(Scheme 4) with 69% yield. This result was in accordance with tained (Scheme 5 and Table 1, entry 4). In particular, a large
what reported in our previous work using a Au cathode (but amount (48%) of the brominated alkyne 3a was isolated, along
with a much lower current efficiency – probably due to side with traces of hydrogenated alkene 4a. In order to confirm the
Table 1: Electrochemical synthesis of 2-ethynylnaphthalene (2a). Electrolysis conditions optimization (Scheme 5).a
entry
cathode
E or Ib
Fc
products (%)d
2a
3a
4a
5a
1e
2
Pt
Pt
Pt
Pt
Pt
Pt
Pt
GC
Ag
Ag
Pt
Pt
−1.75 V
−2.00 V
−2.00 V
−1.75 V
−1.75 V
10 mA/cm2
5 mA/cm2
−1.70 V
−1.80 V
−2.10 V
−2.20 V
−2.20 V
6.0
3.0
4.0
1.5
2.3
3.0
3.0
0.6
3.0
3.0
2.0
3.0
69
80
81
25
27
46
29
5
–
–
–
–
traces
traces
3
–
traces
–
4
48
–
4
traces
5f
6
–
58g
–
–
41
39
–
7
–
–
8
7
7
9
72
65
7
–
6
–
10
11h
12h
–
15
–
2
89
38
–
43
–
–
aElectrolysis conditions: divided cell, 5.0 mL of DMF (catholyte)/0.1 mol dm−3 Et4NBF4 containing 1a (0.5 mmol), rt, N2 atmosphere. Anolyte: 2.0 mL
same solvent. Working electrode: as in Table; anode: Pt; reference electrode: modified SCE (see Supporting Information File 1). The electrolyses
were stopped after total consumption of starting 1a. bControlled potential electrolyses: working potential E (Volts) reported vs SCE. Controlled current
electrolyses: working current density I (mA/cm2) reported. cAmount of charge: number of Faradays. dIsolated yields, with respect to starting 1a.
eACN instead of DMF as solvent. f3 Equivalents of acetic acid were present in the catholyte during electrolysis. gMixture of isomers: Z/E = 69:31.
hNaClO4 instead of Et4NBF4 as supporting electrolyte.
Scheme 5: Possible products from the electrolysis of 2-(2,2-dibromovinyl)naphthalene (1a).
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effect of the presence of a proton donor, acetic acid was added of 2-(2,2-dibromovinyl)naphthalene (1a), irrespective of the
to the solution and the electrolysis was carried out at the first solvent and working electrode material. This renders impos-
cathodic peak potential (Table 1, entry 5). After 2.3 F (total sible a selective cathodic reduction of 1a in the presence of 3a.
consumption of starting material), the alkyne 2a was isolated in The voltammetric behavior of 2-ethynylnaphthalene (2a) shows
27% yield, while the major product was bromoalkene 5a (mix- only one reduction peak at a potential that is quite more nega-
ture of Z and E isomers) in 58% yield. This result is very simi- tive than the first cathodic peak of 1a and 3a, respectively, and
lar to what we reported in our previous work [25].
corresponding to the third reduction peak of 1a and to the
second of 3a. Also in this case the potential value is indepen-
Also the electrochemical methodology has a dramatic effect on dent of the solvent and working electrode material. This voltam-
the products of the cathodic reduction of 1a. In fact, carrying metric analysis shows that the cathodic reduction of both 1a and
out the electrolysis under controlled current conditions (Table 1, 3a could lead to the formation of the desired alkyne 2a.
entry 6) equimolar amounts of desired alkyne 2a and of vinyl
derivative 4a (Scheme 5) were obtained when a current density To ascertain this hypothesis and to get information on the nature
of 10 mA/cm2 was used, while lowering the current density to of the intermediates of the electrochemical process, we carried
5 mA/cm2 did not alter significantly the reaction outcome out the electrosynthesis under the optimized experimental
(Table 1, entry 6 vs 7).
conditions reported in Table 1, entry 2, analyzing the catholyte
during the electrolysis. The yields of electrolysis products 2a
It is well known that the electrode material could influence the and 3a were reported as a function of the number of Faraday
outcome of an electrosynthesis, so we carried out electrolyses of (Figure 2).
1a using a glassy carbon cathode (Table 1, entry 8) and a silver
cathode (Table 1, entry 9). In both cases the working potential
was that of the first reduction wave. In the case of glassy car-
bon, the electrolysis could not be terminated as the current flow
stopped very early [30]. When a silver cathode was used, a
good yield of desired alkyne 2a was obtained (72%), along with
a small amount of hydrogenated alkene 4a (6%). In order to
increase the yield of alkyne 2a (and as 2a reduction potential is
much more negative, vide infra), we carried out a cathodic
reduction of 1a on a silver cathode at the second reduction wave
potential (Table 1, entry 10). In this last case, the selectivity of
the reaction dropped and a notable amount of hydrogenated
alkene 4a was obtained (15%), along with a lower yield of
alkyne 2a (65%).
The effect of a different supporting electrolyte was evaluated by
substitution of Et4NBF4 with NaClO4. Also in this case the
electrolysis was stopped after the complete consumption of
starting 1a (Table 1, entry 11). The change in supporting elec-
trolyte led to a complete change in products. In fact, a very high
Figure 2: Variation of the amounts of 1a, 2a, and 3a with the number
of Faradays of 1a.
yield of 2-(bromoethynyl)naphthalene (3a) was obtained (89%),
along with only 7% of 2-ethynylnaphthalene (2a) after 2.0 F.
Increasing the consumed charge to 3.0 F under the same experi-
mental conditions, an equimolar mixture of bromoalkyne 3a The results of this last investigation (curves reported in
and alkyne 2a was obtained, confirming the possibility of ob- Figure 2) show that i) the concentration of 1a decreases and that
taining 2a by cathodic reduction of 3a (Table 1, entry 11 vs 12). of 2a increases with increasing charge; ii) dibromoalkene 1a is
completely reduced after a consumption of 2.0 F, i.e., a value of
In order to better understand the electrochemical behavior of charge near the theoretical value for the bielectronic reduction
dibromoalkene 1a, we carried out the voltammetric analysis of of a C–Br bond; iii) after a consumption of 2.0 F the yield of
all isolated products (see Supporting Information File 1). The alkyne 2a is 40% versus a yield of 80% after 3.0 F; iv) the anal-
first cathodic peak potential of 2-(bromoethynyl)naphthalene ysis of the solution during the electrolysis shows the presence of
(3a, Scheme 5) is very close to the first cathodic peak potential bromoalkyne 3a.
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Beilstein J. Org. Chem. 2018, 14, 891–899.
The concentration of 3a initially increases and subsequently Bromoalkyne 3a then can be reduced at the electrode to yield
decreases upon increasing the charge; bromoalkyne 3a is absent alkyne 2a. The presence of a proton donor (Table 1, entry 5) in-
in the final solution. The maximum yield of 3a, close to 50%, is creases the yield of 5a and substitutes 1a (as proton donor) in
reached after the consumption of about 1.5 F.
reaction 2 (Scheme 6).
Bromoalkyne 3a and alkyne 2a seem to be strictly related. In The anion generated by cathodic reduction of dibromoalkene 1a
fact, the increase of 3a corresponds to the decrease of starting (Scheme 2, reaction 1) can also eliminate bromide (as reported
1a, while the subsequent decrease of 3a corresponds to the in literature [31]), yielding the corresponding carbene
increase of 2a. In addition the analysis of the electrolyzed solu- (Scheme 2, reaction 2). This carbene can undergo a rearrange-
tions shows the presence of only a trace amount of vinyl bro- ment to yield alkyne 2a. According to the mechanism shown in
mide 5a. Note that vinyl bromide 5a is cathodically active at the Scheme 6, the formation of bromoalkyne 3a competes with the
working potential (see Supporting Information File 1). The formation of 2a in reaction 2 and its rate of formation is compa-
overall analysis allows suggesting a mechanistic hypothesis rable to that of 2a. Since its reduction potential is close to that
(Scheme 6).
of 1a (see Supporting Information File 1, Table S1 and Figure
S2), it is further reduced to the alkyne 2a (reaction 3 in
The bielectronic cathodic reduction of dibromoalkene 1a leads Scheme 6) during the electrolysis.
to the cleavage of one C–Br bond and the formation of the cor-
responding vinyl anion (Scheme 6, reaction 1). An equilibrium The various possible ways described in Scheme 6 are highly
of proton exchange between this electrogenerated carbanion and influenced by the reaction conditions. When the supporting
parent 1a yields vinylbromide 5a and a second vinyl anion electrolyte is NaClO4 instead of Et4NBF4, a different mecha-
(Scheme 2, reaction 2), which is converted to bromoalkyne 3a nism seems to be operative. In fact, following reactions (1) and
by bromide elimination. Vinyl bromide 5a can be cathodically (2) in Scheme 6, a maximum yield of 50% of 3a can be ob-
reduced to 2-vinylnaphthalene (4a) or eliminate HBr to yield tained. It is thus possible that when using NaClO4 an electro-
alkyne 2a.
generated base (OH−) is formed, due to the reduction of water
Scheme 6: Mechanistic hypothesis for the synthesis of alkyne 2a and bromoalkyne 3a from 2-(2,2-dibromovinyl)naphthalene (1a). Alkene configura-
tions are not defined.
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Beilstein J. Org. Chem. 2018, 14, 891–899.
and this base converts 1a to 3a. In fact, the Na+ cation is highly
hydrophilic while the Et4N+ cation is hydrophobic. Thus, in
DMF/NaClO4 the double layer would be constituted by the
strongly solvated Na+(H2O)n, while in DMF/Et4NBF4, the
double layer would be free of water. On Pt, a low hydrogen
overvoltage material, it is then conceivable that the reduction of
water to dihydrogen and hydroxide anions would be faster than
the reduction of 1a. The overall reaction would be a one-elec-
tron process catalyzed by water reduction (Scheme 7) [32].
Scheme 9: Electrochemical synthesis of 1-ethynyl-4-methoxybenzene
(2c).
towards different products simply by changing the electrolysis
It is thus possible by selecting the electrolysis conditions to parameters (potential, solvent, supporting electrolyte, amount of
synthesize selectively 2-ethynylnaphthalene (2a, Table 1, charge, additives, etc.) and making use of electrons (as green,
entry 2) or 2-(bromoethynyl)naphthalene (3a, Table 1, entry 11) cheap, no byproduct-forming reagents) renders electrosynthesis
in high yields.
attractive for organic chemists.
Finally, to test the general applicability of the proposed electro- In particular, this work reported the selective synthesis of
chemical methodology, we submitted to electrolysis (under the 2-ethynylnaphthalene or 2-(bromoethynyl)naphthalene in high
optimized conditions reported in Table 1, entry 2), 3-(2,2- yields by the cathodic reduction of 2-(2,2-dibromovinyl)naph-
dibromovinyl)-9-ethyl-9H-carbazole (1b, Scheme 8). In fact, thalene. The electrolyses were carried out in DMF solution
the corresponding alkyne 2b is an important intermediate in the (Pt cathode) under potentiostatic conditions; if the potential was
synthesis of molecules for organic electronics (e.g., organic fixed at −2.00 V (vs SCE) and the supporting electrolyte was
light-emitting diodes [33] and organic field-effect transistors Et4NBF4, and 2-ethynylnaphthalene was obtained in 80% yield
[34]). The voltammetric analysis showed a behavior similar to after 3.0 F, while using NaClO4 as salt and a potential of
that of 1a (see Supporting Information File 1) and thus the elec- −2.20 V 2-(bromoethynyl)naphthalene was obtained in
trolysis was carried out at the second wave potential. 9-Ethyl-3- 89% yield after 2.0 F. We also demonstrated that 2-(bromo-
ethynyl-9H-carbazole (2b) was obtained in 77% yield.
ethynyl)naphthalene can be cathodically converted to 2-ethynyl-
naphthalene. The extension of the method to two other sub-
Similarly, when starting from 1-(2,2-dibromovinyl)-4-methoxy- strates was successfully demonstrated. This methodology
benzene (1c), the corresponding terminal alkyne 2c was ob- allows carrying out the second step of the Corey–Fuchs reac-
tained in 62% yield (Scheme 9).
tion under milder experimental conditions.
Conclusion
Experimental
The electrochemical methodology is shown to be a useful tool Electrolyses. Constant potential or current electrolyses were
in organic synthesis. The possibility to direct the reaction performed under a nitrogen atmosphere at 25 °C using an Amel
Scheme 7: Possible reaction using NaClO4 as supporting electrolyte.
Scheme 8: Electrochemical synthesis of 9-ethyl-3-ethynyl-9H-carbazole (2b).
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Beilstein J. Org. Chem. 2018, 14, 891–899.
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ORCID® iDs
Fabiana Pandolfi - https://orcid.org/0000-0002-5972-851X
Isabella Chiarotto - https://orcid.org/0000-0001-7165-3226
Marta Feroci - https://orcid.org/0000-0002-3673-6509
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